The invention relates to a fuel assembly for a nuclear reactor of the type comprising fuel elements (generally constituted by nuclear fuel contained in a sheath) and a structure or framework constituted by parallel tie rods fixed to two end parts between which are distributed grids which cooperate with the end parts to hold the fuel elements in a regular network, generally square, of which some lattice sites are occupied by the tie rods.
Such assemblies are widely used in light water power reactors. It has already been proposed (French Pat. No. 2,368,785) to fix one of the end parts removably to tie rods so as to be able to remove it, then to extract at least some of the fuel elements, for example to examine or replace them. This solution has the drawback that, even to replace a single fuel element, it is necessary to dismount the end part, then to remount it, which constitutes a long and delicate operation.
Now it may be desirable to replace, in an assembly, a single fuel element. It is known, for example, that it may be useful to withdraw a fuel element to cause it to undergo destructive examination or to introduce it into a test loop. It is also known that it may be useful to temporarily place a fuel element containing a burnable poison, such as Gd.sub.2 O.sub.3 or B.sub.4 C, in a network of fuel elements occupied by fuel material.
In addition, the construction of fuel assemblies provided to permit the replacement of at least one fuel element situated at a pre-determined location in the network enables a problem encountered in certain nuclear power stations to be easily solved. As a result of hydrodynamic phenomena, such as the presence of violent jets of water directed towards certain fuel elements through the seals of the baffle surrounding the core the reactor, these fuel elements are subjected to vibrations, a source of rupture.
The invention proposes to substitute, at these locations, strong elements, for example of steel, for the fuel elements. When the fuel assemblies are redistributed in the course of recharging core, the steel elements can be replaced by elements of an other type, for example fuel elements, to improve the neutron distribution in the core.
Accordingly, the invention provides in particular an assembly of the above-defined type in which, in at least one of the nodes of the element network, dismountable retaining means for the plug of the fuel elements are provided on one of the end parts, said means comprising a threaded connection actuatable without transmission of considerable torque to the sheath of the fuel element, and enabling the fuel element to be extracted across the part.
Fuel assemblies have already been proposed in which the fuel elements are screwed into the lower part and held elastically with respect to the upper part. To enable the dismounting of certain fuel elements, the latter are provided with a removable member engaging in the upper grid and enabling the fuel element to be extracted. This solution, in which all the fuel elements are held on the lower plate, has the serious drawback of requiring the application of a twisting torque on the fuel elements during their positioning and their removal (French Pat. No. 2,131,857).
This drawback is completely avoided in the case of the invention, applied to a fuel assembly where the intermediate grids participate in holding the fuel elements at the same time as centering them and where the possible twisting torques necessary to act on the threaded connection transmitted by means of the sheath are very highly attenuated.
The invention is capable of numerous embodiments. In particular the means may comprise a sleeve independent of the fuel elements, fastenable to the fuel element or the part by the threaded connection. In an other embodiment, the threaded connection is formed on an extension of the plug which can be screwed into the part, this extension being provided with means accessible from the outside of the assembly to receive a screwing and unscrewing tool.
The invention will be better understood on reading the description which follows of particular embodiments, given by way of non-limiting examples.
The description refers to the accompanying drawings.